Precipitation Characteristics,microstructure And Mechanical Properties Of Al-Mg-Si-?Cu? Alloys Processed By High Pressure Torsion

With the development trends of energy saving and environmental protection in the world,the Al-Mg-Si-?Cu?alloys?6xxx-series?with medium strength,light weight,good corrosion resistance,excellent formability,recyclability,weldability and surface quality are widely used in aerospace,vehicles,construction and other fields.The strength of these alloys can be effectively improved through traditional strengthening method utilizing aging treatment to form a great quantity of nano-scale metastable precipitates within the plastic matrix which hinder dislocation motion due to pinning roles.With the expansion of application fields and scale,how to further improve alloys properties and fully broaden potential applications has become the focus of research in various countries.In recent decades,high pressure torsion?HPT?processing has drawn many an interest from researchers as a promising technique to refine grains and obtain excellent mechanical properties utilizing torsional deformation under extremely high loads.Numbers of experiments show that HPT processing can produce nano-structured materials with grain sizes of less than 100 nm,and obtain 200%³00%increment in strength over undeformed materials.However,low plasticity and toughness of HPTed materials has become an important obstacle to the development of HPT.What counts for the future engineering application is to find out the suitable process combined with HPT deformation to achieve high strength and high toughness simultaneously.Among the many strengthening and toughening mechanisms of aluminum alloys,precipitation strengthening is one of the most effective ways.Therefore,by introducing high-density nanoscale precipitates before deformation,followed by HPT,the precipitation rules,strength and toughness of Al-Mg-Si-?Cu?alloys under different processing conditions are studied to obtain high-strength and high-toughness 6xxx series aluminum alloys.In this study,commercial hot-extruded Al-Mg-Si-?Cu?alloys are selected as the basic alloys.By utilizing hardness testing,tensile testing,differential scanning calorimetry?DSC?,X-ray diffraction?XRD?and transmission electron microscope?TEM?,the effects of HPT on the precipitation behavior,microstructure and mechanical properties of Al-Mg-Si-?Cu?alloys are systematically studied.The main conclusions include:?1?DSC thermal analysis results show that the as-extruded Al-Mg-Si-?Cu?alloys are in peak-aging state before hot extrusion,and the main precipitates dispersed in the materials are?"phases.For 6013 aluminum alloys,due to the higher Cu content?0.6wt.%-1.1 wt.%?,the exothermic peaks of precipitates shift to relative low temperature region and the corresponding activation energy decreases by 2%to 33%,which means a accelerating precipitation kinetics.After HPT deformation,the severe shear strain promotes the resolution of metastable phase?"and reprecipitation of equilibrium phase?in the Al-Mg-Si-?Cu?alloys.?2?XRD results show that the Al-Mg-Si-?Cu?alloys grow favorably and orderly in?111?direction after hot extrusion.As the degree of deformation increases,the?111?preferred orientation weakens and the?200?orientation strengthens.Diffraction peaks get broadened and shifted to high and low angles at the same time for both lattice distortion and residual stress in the alloys.After HPT with different turns of rotation,the grains of the alloys are sharply refined,together with the significantly increased dislocation density.The average grain size and dislocation density after deformation reach 46 nm⁹6 nm and 1.46×10¹44 m^-23.85×10¹44 m^-2,respectively.?3?TEM analysis confirms the peak-aging state of the as-extruded Al-Mg-Si-?Cu?alloys.After hot extrusion,the grains are broken in different degrees and the grain boundaries are flat.Besides,there exist needle-like?"phases,lath-like L phases,Q?phases and a small amount of Si dispersed in the grains,accompanied with few dislocations.The average length of precipitates in the 6013 and 6061 aluminum alloys are 15.3±3.1 nm and 7.8±1.4 nm respectively.After 5 turns of HPT deformation,the grain size of 6013 aluminum alloy reaches the ultrafined scale with the average grain size 124 nm.There exist high density dislocations,large numbers of dispersed particles and dislocation cells in the grains,as well as reprecipitated equilibrium phases at the grain boundary.?4?Hardness measurement results show that the hardness of Al-Mg-Si-?Cu?alloys increases significantly after HPT deformation.Meanwhile,with the increase of turns,the hardness gradually increases and tends to be saturated at high turns.The hardness of 6013 and 6061 aluminum alloys reach the saturated value of 204 HV and 175 HV after twisting for 5 and 1 turns of rotation,which are 85%and 64%higher than that of the undeformed alloys?110 HV and 107 HV?.The radial hardness distribution shows that the hardness of 6013 and 6061 aluminum alloy tends to be uniform when the equivalent effect is greater than 5,and the difference of hardness between the center and the edge is 19%³7%.The microstructure for Al-Mg-Si-?Cu?alloys prepared by high pressure torsion cannot achieve complete uniformity.?5?Tensile test results show that the mechanical properties of Al-Mg-Si-?Cu?alloys get higher with the increased turns of rotation.After 5 and 10 turns of rotation,the tensile strength of 6013 and 6061 aluminum alloys increase from 363 MPa and 298MPa to 550 MPa⁵92 MPa and 528 MPa⁶20 MPa;The yield strength increase from260 MPa and 234 MPa to 519 MPa⁵38 MPa and 477 MPa⁵55 MPa while maintaining a good uniform elongation of 10%¹3%.Among them,6061 aluminum alloy has the best comprehensive performance after 5 turns of rotation at room temperature.Its tensile strength,yield strength,uniform elongation and fracture elongation reach 620 MPa,555 MPa,13%and 27%respectively.?6?The high strength,hardness and good ductility of HPT Al-Mg-Si-?Cu?alloys are the results of many strengthening and toughening mechanisms.By introducing aging precipitates into the material and applying high-pressure torsion to the alloy,the composite structure with nano-scale precipitates,high-density dislocations and ultrafined-grains are obtained.The pinning effect of high-density precipitates in the intergranular and grain boundary region increases the deformation resistance and dislocation storage capacity.Consequently,the alloy has both high strength and high toughness.